The arrow is pointing at the pointed papillae covering the penis of Chondrus tournefortianus. I don't know what function the papillae may have.
The picture shows the everted penis of the land snail Chondrus tournefortianus. This hapless pair was mating on a hillside at the outskirts of the city of Kastamonu, Turkey one cold and wet morning in October 2008 when I chanced upon them. Not only did I intrude upon their privacy by photographing them, but I also pulled them apart to see if their mating was anatomically reciprocal.
Remember that pulmonate snails, and that is what these are, carry both male and female; in other words, they are hermaphrodites. Therefore, when they mate, it is possible for each snail in a pair to use its penis to inseminate its partner. In many snail species, mating is indeed anatomically reciprocal*. But in some species only one snail acts as the male and gets to inseminate its partner.
One way to determine if the mating of a pair is anatomically reciprocal is to kill them in copula and then to dissect them. I don’t particularly like killing snails, especially if they are in the process of performing a fundamental evolutionary act, that is, passing on their genes. So instead, I pull them apart while watching them under a microscope or a magnifying glass, if I am in the field, and hoping to see their penises as they are being withdrawn (more about that technique here). Other than one interrupted coitus, no damage is done.
In the case of this particular pair of Chondrus tournefortianus, after I separated them, I saw the penis of only one snail and, therefore, concluded that their mating was anatomically unilateral. The details of this lucky encounter and how it has contributed to our knowledge (or the lack thereof) of the family Enidae, to which this species belongs, have just been published in a short paper of mine in the journal Zoology in the Middle East. You may read it here. The paper also has a photo of the mating snails before I took the matter into my hands.
*Strictly speaking, anatomical reciprocity doesn’t necessarily mean reciprocal insemination; an individual could mate without contributing sperm. Anatomical reciprocity can also be simultaneous or sequential. Yes, I know, it does get complicated.